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Sandglass Timers

When used together with the chip log, a sandglass was used to measure the ship's speed through the water (in knots).

A chip log consists of a wooden board attached to a line (the log-line). The log-line has a number of knots tied in it at a uniform spacing, and the line is wound on a reel to allow it to be paid out easily.

Over time, the chip log was standardized in construction. The shape is a quarter circle, or quadrant, and the log-line is attached to the board with a bridle of three lines connected to the vertex and to the two ends of the quadrant's arc. In order to ensure that the log submerges and is oriented correctly, the bottom of the log is weighted with lead. This provides for more resistance in the water and a more accurate and repeatable reading of speed. The bridle is attached in such a way that a strong tug on the log-line results in one or two of the bridle's lines releasing, allowing the log to be retrieved with relative ease.

Originally, the distance between knots on the log-line was 7 fathoms, or 42 feet, and a 30 second sandglass was used. Later refinements in the length of the nautical mile caused the distance between knots to be changed. Eventually, the distance was set to 47 feet, 3 inches (14.4 meters) and a standard 28 second sandglass timer was used. But as ship speeds increased in the early 19th Century the length of log-line paid out over 28 seconds became excessive so, on ships like the China clippers, which had peak average speeds over 16 knots, a 14 second sandglass was used and the log count was doubled.